1. Field of the Invention
The present invention relates to an ink jet printing method, an ink jet printing system, an ink jet printing apparatus and a control program, all capable of expressing desired grayscale information by printing a print material on a print medium.
2. Description of the Related Art
As a growing number of information processing devices such as personal computers have proliferated in recent years, printing apparatus as an image forming terminal have also been developed and come into wide use. Among a variety of kinds of printing apparatus, an ink jet printing apparatus in particular, which performs printing by ejecting ink from nozzles onto a print medium, such as paper, cloth, plastic sheets and OHP sheets, is now a mainstream printing apparatus for personal use because of its excellent features including the use of a low-noise non-impact printing system, an ability to print at high density and at high speed, an ability to cope with color printing with ease, and low cost.
The advance in the ink jet printing technology has led to a higher print quality, a higher printing speed and lower cost and, in combination with the proliferation of personal computers and digital cameras (including those that can be used as single devices and those that are built into other devices such as mobile phones), has greatly contributed to making the printing apparatus popular even among personal users. With such a prevalence of printing apparatus, there are increasing demands even from personal users for further improvements in print quality. Recent years in particular have seen growing demands for a print system that allows easy home printing of pictures and for a high print quality that equals that of silver salt pictures.
When compared with general silver salt pictures, images formed by the ink jet printing apparatus have a problem of a characteristic graininess. Various countermeasures have been proposed in recent years and many printing apparatus incorporating such measures are also available. For example, there is an ink jet printing apparatus with an ink system which has light cyan and light magenta inks of reduced density in addition to commonly used cyan, magenta, yellow and black inks. With such an ink system, the light cyan or light magenta can be used in areas of lower grayscale level to reduce the graininess; and in areas of high grayscale level, normal cyan and magenta inks are used. This method has realized a wider color reproduction and a smooth tonality.
Another method reduces the graininess by making dots landing on a print medium smaller. To realize this method, a technology is being developed to minimize the size of ink droplets ejected from nozzles arrayed in a print head. In this case, in addition to reducing the ink droplet size, the print head is designed to incorporate a greater number of nozzles at a higher array density to produce a high-resolution image without compromising the printing speed.
While the personal use ink jet printing apparatus is required to be able to produce images of high quality close to that of the silver salt pictures as described above, it is often required to also output normal documents such as texts and tables. In printing such documents it is essential to print them at high speed, rather than at high quality like that of silver salt pictures. Therefore, general ink jet printing apparatus are provided with a plurality of print modes to allow the user to choose a desired mode as required (for example, Japanese Patent Application Laid-open No. 1-281944(1989)).
However, not all technologies developed to improve the image quality can coexist with a print mode that places priority on low cost and high speed printing. For example, in an ink jet printing apparatus that cannot modulate a volume of ink ejected from the nozzles (referred to as an ejection volume), all ink droplets ejected from the nozzles arrayed in the print head are small drops of a fixed volume in order to reduce graininess; and dots formed of the fixed volume of ink are arranged at an appropriate resolution to produce a desired grayscale level (e.g., Japanese Patent No. 03184744). As the ejection volume decreases, the concentration or resolution of printed dots increases to produce a desired grayscale level. Further, the associated means and data processing, though complicated, are fixed to some extent. Therefore, even in a high-speed mode, there is no alternative but to use the fixed means and time-consuming data processing method, making it difficult to produce an image of a desired grayscale at a satisfactory printing speed.